The present invention is directed to solar energy collection devices, and more particularly to a system and method for controlling a field of heliostats used to direct solar energy to a plurality of receivers for power generation.
Various technologies have been explored, developed, and implemented for generating electricity from solar energy. The two principal technologies for converting solar energy into electricity are photovoltaic and thermal means. Photovoltaic “solar cells” convert incoming photons directly into electricity using the photoelectric effect, for example using semiconductors based on silicon (Si) or gallium-arsenide (GaAs). Thermal conversion captures broadband solar radiation in a heat transfer medium (for example, oil, water, or air) and then uses a traditional steam or gas turbine to spin a generator. The simplest systems mount solar energy collectors in a manner that fixes their orientation with respect to the surface of the Earth; however, many technologies used for converting solar energy into electricity actively track the motion of the Sun. Tracking refers to following the motion of the Sun as it moves across the sky through the day and how this path changes through the seasons. In particular, systems that concentrate solar radiation to high intensity require some form of active tracking. Very low concentrations (less than two times to about five times the level of incident radiation) can be achieved without using tracking systems. Such systems typically cannot generate power throughout the day. Low concentration is also incompatible with thermal power generation means.
As used herein, Concentrated Solar Power (CSP) technologies are technologies that use Sun-tracking optical elements to concentrate solar radiation. Although, as noted above, some technologies implement and utilize low concentration factors, systems utilizing such technologies are not the subject of this invention and are thus excluded herein from the definition of CSP for conciseness. Concentrating solar power systems collect solar radiation at its standard irradiance level of about one thousand watts per square meter (W/m2) and concentrate it to a higher intensity at its point of use. In such systems, the element or subsystem that collects solar energy is called the collector and the element or subsystem that concentrates solar energy is called the concentrator. In many systems, the collection and concentration functions are performed by a single subsystem. In such systems, the integrated element or subsystem may be referred to as either a collector or a concentrator, interchangeably. This element or subsystem may also be referred to as the primary optical element or primary optical subsystem of the system. The collecting and concentrating elements might be reflective (mirrors) or refractive (lenses), and a system might use a combination of the two.
In concentrating solar systems, the final destination of the solar radiation is generally referred to as a receiver. The receiver might perform direct conversion to electricity (for example, using photovoltaic means), might capture the energy as heat, or might use the energy for other purposes (for example, lighting). In many systems, a secondary optical element or secondary optical subsystem is collocated with the receiver and might be considered to be part of a receiver assembly. This secondary optic might perform concentration, beam homogenization, or other functions. The higher intensity created by concentration can be desired to generate higher temperature at the receiver, produce higher photovoltaic cell efficiency, or require smaller receivers.
There are three main CSP technologies being developed internationally: parabolic-trough power plants, parabolic dish systems, and central receiver systems. CSP technologies have been constructed in various sizes, including small multi-kilowatt (kW) systems and large power stations of tens of megawatts (MW). Such power stations have provided the cheapest electricity to be generated using solar energy.
Parabolic-trough power plants utilize large fields of Sun-tracking, linear parabolic trough collectors that concentrate energy onto steel tubes through which circulates a heat transfer fluid (HTF). This high temperature fluid is pumped through heat exchangers to generate steam of up to 400° C. that are used to power a conventional steam turbine to produce electricity. These systems achieve moderate levels of concentration, typically about ten times to about one-hundred times the level of incident radiation. This type of concentration, sometimes referred to as 2-D (two-dimensional) concentration, is thermodynamically limited to just over two-hundred times the level of incident radiation. Typically arranged on north-south lines, the collectors rotate around a north-south axis to track the Sun from east to west during the day to ensure that the Sun is continuously focused on the linear receiver tubes.
A variant on the parabolic trough concept uses a Fresnel arrangement of long, linear reflective elements to synthesize the optical function of the parabolic trough reflector. The linear reflective elements are rotated along their long axes to effect the pointing control function that is equivalent to rotating the parabolic trough about its long axis. These systems are referred to as linear Fresnel reflectors.
For various reasons, higher levels of incident radiation concentration then can be achieved by 2-D concentration (for example, as used in parabolic-trough power plants) might be necessary to be economically attractive. These reasons might include higher efficiency from higher temperature fluid feeding a steam plant or lower receiver area for high-cost multi-junction solar cells that can handle the higher intensity illumination.
Concentration ratios of as high as three-thousand times the level of incident radiation have been described in the literature. Concentration levels above about two-hundred times the level of incident radiation generally require using three-dimensional (3-D) concentration. A parabolic dish reflector provides one example of a 3-D concentrator. To provide the high levels of concentration that the parabolic dish can provide, the dish must be continuously reoriented to track the motion of the Sun across the sky. This approach has been applied to dishes of up to about ten meters in diameter. At very large sizes, a single large reflector becomes increasingly difficult to build or to control with the requisite precision.
As shown in FIG. 1, the central receiver architecture 100, also known as “power tower architecture,” includes a system of hundreds or thousands of large, two-axis reflector systems that track the Sun 190 and reflect incident radiation to a common receiving location 110. Each tracking reflector system is referred to herein as a “heliostat” 150, which is a device in which one or more mirrors are moved to direct solar radiation in a specified direction for a period of time. The set of all heliostats in a solar energy collection system is referred to as a “heliostat field,” which might be organized as one or more heliostat subfields 162, 164, 166, 168. A “receiver” (a device for capturing solar radiation and converting it into another form of energy) is generally placed at the receiving location 110. The receiving location 110 is placed well above the heliostat field, typically on a tower so as to elevate the receiving location above the ground level of the heliostat field and to help prevent interference between the reflected radiation and other heliostats. Using heliostats, a large number of reflectors can be made to track the Sun's motion during the course of a single day and accommodate the Sun's changing path from day to day through the seasons. This approach can provide very high levels of concentration of incident radiation.
The central receiver architecture uses a relatively large number of heliostats (from hundreds to thousands), typically arranged in concentric rows 172, 174, 176, 178 to direct incident radiation from the Sun 190 to a single “receiver” (a device for capturing solar radiation and converting it into another form of energy), resulting in a typical concentration ratio between 500:1 and 1000:1, though lower and higher ratios have been discussed and/or implemented. The heliostats are generally implemented as very large structures so as to control heliostat field costs by minimizing the number of units built and improving the ratio of reflector area relative to such items as control motors. The size of the structures results in scaling issues related to bending, wind-loading, and construction.
Referring to FIG. 2, the prior art contemplates an approach for implementing hundreds or thousands of megawatts of generating capacity wherein a large land area would be populated with a number of central receiver systems 210, 220, 230, 240. In such an approach, the design of a single power tower 211, 221, 231, 241, associated generating and support equipment 212, 222, 232, 242, and associated heliostat field 217, 227, 237, 247 is replicated multiple times. Further, the prior art contemplates designing each central receiver and associated heliostat field to provide several tens of megawatts of capacity apiece, as is considered necessary to reduce per-plant development costs (for example, environmental impact reports, development planning, common parts manufacturing, etc.) and operations and maintenance costs (by sharing personnel, equipment and spares between multiple systems). While the prior art includes significant approaches for optimizing a single central receiver power station, the replication of a single design across large land areas, as illustrated in FIG. 2, obscures the fact that this creates a new, larger meta-system whose overall performance can be further optimized. Heretofore known solar energy collection systems do not provide a method for optimization across the several heliostat fields.
Referring to FIG. 3, a prior art central receiver system uses a plurality of heliostats 350 to direct and concentrate incident radiation 392 to a single central receiver 310. The receiver 310 absorbs concentrated solar radiation 394, converts it to heat, and uses it to heat an HTF 326, such as a synthetic oil or molten salt, to a specified temperature. The degree of heating of and heat transfer to the receiver depends on several design factors, such as heliostat field size, receiver shape and size, HTF limitations, thermodynamic limitations, and end-use application. A power generation system 320 for converting the heat generated by the solar portion of the system may include a tank 328 for storing hot HTF, a steam generator 342 that uses the HTF to generate high quality steam, and a tank 324 for storing cooler HTF after heat has been extracted for steam generation before returning the HTF via pipes 322 to the receiver. The output of the steam generator 342 is fed to a conventional electric power generation system 340 that may include a steam turbine 346 and a condenser 348 that feeds water and/or used steam back to the steam generator. Electricity may be conducted to a power grid 360 by conventional transmission systems 362. The most common end-use application for central receiver technology is a Rankine power cycle, although the technology can also be used as the heat source for other, more efficient cycles such as Brayton/combined cycle as well as high-temperature process steam. Various investigations have also explored the use of photovoltaic receivers on a small scale, but no large system is known to have been implemented using this approach.
Referring to FIG. 4, the reflecting element of a single heliostat 400 can approximate a curved surface by using a number of smaller mirrors 412, 414, 416. Each individual heliostat is then controlled in such a way that its reflected radiation is directed to a receiver. A heliostat used at 35° latitude must generally have enough range of motion to accommodate the motion of the Sun from 65° E (summer solstice sunrise at 35° latitude) to 65° W (summer solstice sunset) or a total of 310° of azimuth angle, plus almost 80° of motion in elevation. Heliostats capable of the required range of motion are generally built using an azimuth motor and bearing plus an elevation motor and bearing. Such trackers are called “az-el” mounts.
As shown in FIG. 4, the reflecting element or surface of the heliostat is configured from one or more mirrors 412, 414, 416 that may be secured by a vertical fixing mechanism 420 and brackets 422 and/or horizontal brackets to combine the individual mirrors into a reflector assembly, creating essentially a single reflecting surface. The heliostat may include a standard, pole or other supporting structure 430 having a plate or other anchoring device 432 for securing the supporting structure to the earth or other surface, such as a rooftop. Specialized electronics, such as motor controllers, computers and interfaces with external sensors, may be included within a section 450 of the heliostat supporting structure. These electronics are operably connected to a dual-axis actuator 460 (for example, “az-el” mounts) that is mechanically connected to a horizontal bar 440 or other mechanism that is connected to the brackets 420 of the heliostat reflector assembly. By way of example and not as a limitation, other solar systems having one or more collectors for receiving and using radiant energy from the Sun are found in U.S. Pat. Nos. 4,276,872; 4,227,513; 4,137,897; 5,899,199; 6,131,565; 4,102,326; and RE 30,960, the contents of each of which are hereby incorporated herein by reference.
Three types of solar radiation 500 are illustrated in FIG. 5. Whereas flat-panel photovoltaic devices can utilize both diffuse radiation 580 and ground-reflected radiation 520, concentrating optics 550 generally only use incident solar radiation 510 that travels directly from the Sun 590 through the atmosphere to the optics. This radiation is termed “direct beam radiation” or “direct insolation.” Other solar radiation is not available to such a collection system, such as reflected radiation 530, scattered radiation 570, or absorbed radiation 560 all caused by clouds, dust, and other phenomena of the atmosphere 560.
The utilization of solar radiation in central receiver systems requires attention to losses caused by two effects, termed cosine effects and shadowing and blockage effects (see FIGS. 14-22). Cosine loss is the effective loss of aperture area of a heliostat reflector due to tilting the reflector away from the Sun. A heliostat “shadows” another heliostat when a first heliostat intercepts the Sun's rays before it reaches a second heliostat. A heliostat “blocks” another heliostat when a first heliostat prevents the Sun's rays from being further used in the system. Shadowing and blockage decrease utilization of heliostats and thus have indirect costs. The typical spacing of heliostats in prior art central receiver systems to address shadowing and blocking leads to relatively low land-use efficiency. The typical spacing in such systems also leads to significant heliostat distances, which in turn also increases losses due to atmospheric path-length absorption and beam spread.
Cost-effective implementation of heliostat fields in the prior art has addressed cosine losses by biasing heliostat placement toward the north of a central receiver (in the northern hemisphere), sometimes at significant distances and with consequent atmospheric transmission losses, and by avoiding placing many heliostats to the south of the central receiver. However, cosine losses incurred in the early morning and later afternoon have been heretofore considered unavoidable. The prior art has addressed shadowing and blocking through the use of optimized heliostat field layouts that minimize these effects to increase the cost-effectiveness of the heliostat field. This approach results from the preference for acute-angle reflection to avoid cosine losses but results in relatively low land area utilization.
By way of definition, when the angle between the incident light ray and reflected light ray is less than perpendicular (90°), the phenomenon is referred to herein as an “acute-angle reflection” (see FIG. 15). Conversely, when the angle between the incident light ray and reflected light ray is greater than perpendicular (90°), the phenomenon may be termed an “oblique-angle reflection (see FIG. 16).
Accordingly there is a need for, and what was heretofore unavailable, a method and apparatus for further reducing cosine losses and shadowing and blocking effects in heliostat fields used for solar power concentration, and for increasing the land use efficiency of heliostat fields. The present invention solves these and other needs.